The present invention relates to improvements in or relating to receivers, and is more particularly concerned with direct conversion receivers.
Direct conversion receivers have a major advantage over conventional superheterodyne architecture because the intermediate frequency (IF) is zero, and there is no need for image filtering. Furthermore, spurious products produced by the mixer (due to the harmonics of the radio frequency (RF) and the local oscillator) fold into DC and hence RF filtering is not required. These features associated with direct conversion are extremely useful for broadband applications such as xe2x80x9csoftware radioxe2x80x9d or multicarrier receivers.
One major problem, however, with direct conversion is the direct detection which is caused by the non-linearity of mixers used in the conversion. The even order terms demodulate the envelope of any interfering signals within the RF bandwidth, and these then fold into the I/Q baseband and become inseparable from the wanted signal. Because of this problem, direct conversion receivers are only suitable for applications where the interference is low, for example, in satellite receivers.
It is therefore an object of the present invention to provide an improved direct conversion receiver which overcomes the problems mentioned above.
In accordance with one aspect of the present invention, there is provided a method for converting an input radio frequency signal to an output signal, the method comprising the steps of:
a) receiving the input signal;
b) applying the received signal to at least a first and a second processing path;
c) modulating the received signal in each processing path to produce modulated signals having different phases; and
d) combining the modulated signals from each path to provide the output signal;
characterised in that the method further comprises the step of applying correction to at least one of the processing paths prior to step d).
Advantageously, the modulated signals are digitised prior to correction.
Correction is applied by means of a cancellation loop driven by the output signal, the cancellation loop determining a power value for the output signal, using the power value to generate a correction signal, and using the correction signal to adjust the amplitude and phase of the modulated signal. Additionally, step a) may comprise adding a pilot signal to the received signal for calibrating differential non-linearity between processing paths, the pilot signal being filtered from the output signal prior to determining the power value.
In a preferred embodiment, step d) comprises subtracting the processing paths.
In accordance with another aspect of the present invention, there is provided a receiver comprising:
means for receiving an input signal;
means for applying the received signal to at least a first and a second processing path;
modulation means for modulating the signals in each signal path to produce modulated signals having different phases; and
combining means for combining the modulated signals from each path to provide an output signal;
characterised in that the receiver further comprises correction means for applying a correction to at least one of the processing paths prior to combination.